At some point during the 3rd century BC, Xiang Yu, the leader of a rebellion against China's First Emperor, Qin Shi Huang, succeeded in getting access to the outer chambers of the imperial tomb.

Inside, thousands of terracotta soldiers stood vigil, each one minutely different, arrayed in battle formation to protect the mausoleum.

The First Emperor, who had succeeded in unifying China's warring states, conscripted some 700,000 convicts over 38 years to build the tomb at Lishan mountain, near modern-day Xi'an.

Xiang, however, was bent on revenge. "He probably managed to enter the tomb with the help of a guide. He might have taken a Qin prisoner," said Shen Maosheng, the head of the archaeological team working on the current excavation.

"Xiang and his soldiers then stole weapons from inside and smashed many Terracotta warriors. After that, they set fire to the chambers".

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Mr Shen said it was not clear how Xiang had carried out the arson, but that the ash at the site indicated a very fierce flame. He added that the fire seems to have blown out by itself, but that the damage had been extreme.

Today, four main pits of the tomb, which was stumbled over by farmers digging a well in 1974, have been excavated, but much is yet to be discovered. Archaeologists are currently working on the third major excavation, which began in 2009.

In the first and second pits, and in tombs containing stone armour and a troupe of terracotta acrobats, several figures were "severely burnt and sunk into the ground where they were found," said Mr Shen. One of the destroyed figures, which has just been discovered, was a headless, eight foot-tall warrior with a huge lacquered leather shield.

Mr Shen said Xiang was angry at the loss of his men to Qin soldiers. "The Qin army had killed 400,000 soldiers of one of Xiang's allies, and that might be the main motive," he said. "It was revenge, not the need for weapons, that was the likely trigger," he added.

Born in what is today Jiangsu province, Xiang died at the age of 30. Before that, he was famed for his boldness, arrogance, and bad temper.

In one major battle against the Qin forces, he ordered his soldiers to break their cooking pots and scupper their boats, as a sign of their determination not to retreat.

Traces of fire damage have been recorded at the site before, but the revelation that it was a man-made arson is "one of the major achievements of the third excavation," said Mr Shen. Previously, it was suspected that a build-up of gas in the tomb had spontaneously ignited.

"The damage to the tomb is too serious for common grave robbers, and you can see from the burned ropes that the fire was set in ancient times," he added. Archaeologists also found that many of the terracotta figures had been smashed before the fire.

They also found evidence that the Terracotta Army had a "wing guard" to protect its flanks from an attack from the side and unearthed 41 figures that were "very different" from soldiers and may have been intended to entertain the emperor in the afterlife.